An electronic control unit in which high-heat-generating devices, such as a switching device, a relay, a coil, and a capacitor, are mounted on a circuit board is known. The high-heat-generating device generates a lot of heat when operating. For example, in JP-A-2012-59759 corresponding to US 2012/0057318A1, a switching device, a relay, a coil, a capacitor, and a shunt resistor are mounted on the opposite side of a circuit board from a heat-dissipating member.
In the electronic control unit disclosed in JP-A-2012-59759, a heat-conducting member is located between the heat-dissipating member and part of a region where the switching device, the shunt resistor, and the capacitor are mounted. Accordingly, heat of the switching device, the shunt resistor, and the capacitor is transmitted to the heat-dissipating member through not only the heat-conducting member but also the circuit board. As a result, heat of the switching device, the shunt resistor, and the capacitor may be dissipated inefficiently.
The relay and the coil are mounted on a region separated from the region where the switching device, the shunt resistor, and the capacitor are mounted, so that the relay and the coil are located away from the heat-dissipating member. That is, JP-A-2012-59759 does not consider dissipating heat of the relay and the coil. Therefore, the electronic control unit may be heated by the heat of the relay and the coil.
In particular, the electronic control unit disclosed in JP-A-2012-59759 is used to control a motor of an electric power steering apparatus for a vehicle. When the motor of the electric power steering apparatus is driven and produces assisting-torque to help a driver to steer a vehicle, a large current flows through the motor and the high-heat-generating devices of the electronic control unit. Therefore, when the motor is driven, the high-heat-generating devices generate a lot of heat. For this reason, when an electronic control unit is used to control a motor of an electric power steering apparatus, it is preferable that heat of high-heat-generating devices of the electronic control unit should be dissipated efficiently.